Within the same time zone, the sun is at its zenith at a different time depending on whether one is at the extreme east, at the centre or extreme west of the time zone. There is a time difference of 59 minutes between the two extreme positions.
Moreover, the country in which the user is situated may not be aligned with the official time zone time. This is, for example, the case of Switzerland, which although within the Greenwich time zone, has a one hour time difference with the official time zone time.
Other countries have only one official time but their territory covers several time zones.
Finally, some countries change time according to the season (summer time/winter time).
By way of example, for someone in Neuchâtel (Switzerland) on 23 July, the sun will be at its zenith at 14:38 hours in civil time, namely: 12 hours (time zone time), +2 hours (summer time) −28 minutes (longitude of Neuchâtel: 7°) +6 minutes (difference from running equation of time). Conversely, for someone in London on the same day, the sun will be at its zenith at 13:06 hours in civil time, namely: 12 hours (time zone time) +1 hour (summer time) +0 minutes (longitude of London: 0°) +6 minutes (difference from running equation of time). Yet Neuchâtel and London are in the same time zone.
FIGS. 1, 2 and 3 annexed to this patent application illustrate the prior art differential device to which the universal running equation of time mechanism of the invention applies.
This differential device is described in detail in European Patent Application No 1286233 in the name of Frederic Piguet S. A. Let us recall that FIGS. 1, 2 and 3 annexed to this patent application and taken from the aforementioned European patent application, show, in particular, the equation of time cam 1 whose profile is determined by the difference, for each day of the year, between mean solar time or civil time and true solar time.
Indeed, as is well known, there is a difference between true solar time, which is the time that elapses between two consecutive passages of the sun above the meridian at the same location, and mean solar time or civil time which is the mean duration in a year of all the true solar days. This difference between civil time and true time reaches +14 minutes 22 seconds on 11 February and −16 minutes 23 seconds on 4 November. These values vary very little from year to year.
The equation of time cam 1 is driven in rotation at the rate of one revolution per year from the simple or perpetual date mechanism comprised in the timepiece. Cam 1 carries a month disc 2 which rotates at the same speed than cam 1 and which allows to match the position of said cam 1 to the date indicated by the date mechanism so that the solar time minute hand 4 indicates the exact solar time.
The simple or perpetual date mechanism may be of any known type and will not be described in its entirety here. For a clear understanding, it is sufficient to know that this date mechanism drives equation of time cam 1 at the rate of one complete revolution per year. However, purely for the purpose of illustration, a date wheel set 6 driving a hand 8 which indicates the date (from 1 to 31) is shown. This date wheel set 6 rotates at the rate of one complete revolution per month. It is actuated by the date mechanism via an intermediate return date wheel 10 for reversing the direction of rotation, and a reduction wheel set 12 for reducing the rotational speed from one complete revolution per month to one complete revolution per year.
The solar time minute hand 4 is driven by a differential gear 14 which has as respective inputs a gear train 16 driving a civil time minute hand 18 and a rack 20 which cooperates with equation of time cam 1 (rack 20 is shown in FIG. 1 in both of its end positions, once in a full line and the other time in dot and dash lines). More specifically, as seen in FIG. 1, differential gear 14 includes at least one and preferably two planetary wheels 22 driven by the motion work of the watch movement. These two planetary wheels 22 are capable of rotating on themselves and rolling over the inner toothing 24 of an equation of time wheel 26. The latter also has, on the external periphery thereof, a first toothed sector 28 via which it cooperates with a second toothed sector 30 comprised on one of the ends of rack 20. This rack 20 is subjected to the return action of a spring (not shown) which is fixed to the watch frame and which tends to apply a feeler spindle 32, forming the other end of said rack 20, against the periphery of running equation of time cam 1. The solar time display train includes a pinion 34 placed at the centre of differential gear 14 and carried by an arbour 36. This solar time display pinion 34 meshes with planetary pinions 22. It also carries a display wheel 38 which meshes with a cannon-pinion 40 onto the pipe of which there is driven the solar time minute hand 4. This gear train 38, 40 returns the solar time display to the centre 42 of the watch movement, so that the solar time minute hand 4 is concentric with civil time minute hand 18.
The running equation of time mechanism which has just been described operates as follows.
In the normal operating mode of the watch, equation of time cam 1, equation of time rack 20 and thus equation of time train 26 are immobile.
However, planetary pinions 22 are driven by the watch movement. Thus, they rotate on themselves and roll over the inner toothing 24 of equation of time wheel 26, driving solar time pinion 34 in rotation, which permits the solar time minute hand 4 to rotate in a concomitant manner with civil time minute hand 18. The difference between solar time hand 4 and civil time hand 18 thus remains constant over a period of 24 hours.
Once per day, at around midnight, the running equation of time cam 1 pivots, driven by the date mechanism which changes the date from one day to the following day. At that precise moment, feeler spindle 32, which is in contact with the periphery of cam 1, in turn pivots rack 20. Said rack 20, in pivoting, drives equation of time wheel 26 in rotation. Planetary pinions 22, which are substantially immobile during this brief time interval (they make one complete revolution in one hour), rotate on themselves, driven in rotation by equation of time wheel 26 and in turn drive solar time display pinion 34 so as to precisely set the position of solar time minute hand 4 again.
Thus, the running equation of time mechanism described above can, at any time, display the time difference between mean solar time and true time, by means of a civil time minute hand and a solar time minute hand. This running equation of time mechanism does not, however, indicate the civil time at which the sun is at its zenith according to the position, in terms of longitude, of the user within the time zone.